Important elements for the electric power steering system to receive high evaluation in respect of steering feeling are to move tires (vehicle) linearly in response to an active input torque (to make a turn or K-turn), to provide a direct transmission of road information (frequency band<5 to 10 Hz), and the like.
However, the conventional electric power steering systems offer low steering performance having poor linearity and difficulty in assessing road conditions, so that drivers have an unnatural steering feeling.
Specifically, frictions at torque transmission elements (manual steering gear, reduction gear assembly for steering assist motor, and the like) of the conventional electric power steering system are considered as necessary evil and are set to relatively high values.
At first view, setting the frictions at the torque transmission elements to high values provide the following merits. That is, a handle (steering member) is stabilized against passive input torque (straight-ahead drive, retention of steering angle, turn back of handle), is less susceptible to road noises (unwanted frequency band>5 to 10 Hz) (road noise resistance), and is improved in convergence.
However, if the torque transmission elements have great frictions, the system is decreased in the efficiency of transmitting the active input torque. In addition, the system having the great frictions requires a high power motor or a reduction gear having a high reduction ratio in order to provide a required assist force. This consequently results in the increase of rotor inertia about a steering shaft.
The increase of inertia leads to a lowered response in the transmission of an input steering torque (positive input). What is more, the handle return is also impaired significantly.
As described above, the system having the great friction and inertia is incapable of providing the good steering feeling.
As solutions to these problems, compensation logics (inertia compensation, friction compensation, damping compensation, handle return control and the like) have been proposed and applied to the systems, the logics designed to permit the high friction/high inertia characteristics of the system and requiring various determination conditions for making determinations to make compensations.
Unfortunately, however, these compensation logics not only impair the coordination of the overall system but also entail the following problem. Since the individual compensation logics are independent from one another, it is inevitable that plural compensation outputs are active at all times so as to interfere with one another in actual driving conditions where operating conditions of the system vary in various ways.
As a result, the state-of-the-art electric power steering systems provide the unnatural steering feeling with poor linearity and offers difficulty in assessing the road conditions, so that drivers experience some awkwardness.
That is, the improvement of the steering feeling based on the compensation logics is a kind of stopgap measure, which rather causes the drivers to experience some awkwardness.
Japanese Unexamined Patent Publication No.2003-40120, for example, discloses a technique (hereinafter, referred to as Prior Art 1) wherein an inertia converted to a value about handle-shaft is defined to be 4×10−2 kg·m2 or more and 10×10−2 kg·m2 or less, the inertia determined based on an inertia of a brushless motor and a reduction gear ratio of a reduction gear portion. Prior Art 1 permits the relatively great inertia of 4×10−2 kg·m2 or more in order to suppress the road noises such as kick-back from road surface, and provides an inertia compensation control such as to compensate for the steering feeling degraded by the inertia feeling resulting from the great inertia. That is, Prior Art 1 is designed to compensate for the inertia feeling based on the inertia compensation. This is nothing but a stopgap measure.
Japanese Unexamined Patent Publication No.2001-334948 discloses a technique (hereinafter, referred to as Prior Art 2) wherein a complementary sensitivity function is defined to approximate 1 in a frequency band containing disturbances to be suppressed whereas the complementary sensitivity function is defined to approximate 0 in a frequency band containing disturbances to be transmitted. According to Prior Art 2, as well, the suppression of disturbances is accomplished by permitting the relatively great motor inertia. Specifically, the inertia of the motor is positively utilized for suppressing the unwanted disturbances, while the motor inertia perceived by the driver steering the handle is compensated by a torque control system. When the inertia is increased, resonant frequency of a steering mechanism is decreased. If the relatively great inertia is permitted, there may sometimes arise a problem that even a frequency band of road information as the disturbances to be transmitted must be damped. This makes it impracticable to realize an electric power steering system providing the ease of assessing the road conditions.
Furthermore, Japanese Unexamined Patent Publication No.2001-18822 discloses a technique (hereinafter, referred to as Prior Art 3) wherein determination is made as to whether the disturbances are present in a region where the driver is sensitive to the steering feeling and based on the determination result, a control gain is changed or correction using a non-interactive control correction value or a pulsating torque correction value is provided or not.
Furthermore, Japanese Unexamined Patent Publication No.2003-40128 discloses a technique (hereinafter, referred to as Prior Art 4) wherein the system includes correcting means for correcting a current command value used for controlling the magnetic field of the motor and wherein the current command value for the control of the magnetic field of the motor is corrected when a steering speed is high.
Furthermore, Japanese Unexamined Patent Publication No. H3(1991)-178868 discloses a technique (hereinafter, referred to as Prior Art 5) wherein a viscous-friction compensation value is corrected according to a detected vehicle speed in a manner that the viscous friction is increased when the vehicle speed is high and that the current has such a polarity as to eliminate the viscous friction and a small absolute value when the vehicle speed is low.
The controls provided by Prior Art 3 and 4 involve discontinuous torque variations when the control modes are switched based on the determination result. The correction proposed by Prior Art 5 also involves unnatural torque variations in conjunction with the vehicle speed variations.
Furthermore, Japanese Unexamined Patent Publication No. 2000-238655 (Prior Art 6) discloses a technique wherein a torque ripple of the motor is reduced to 10% or less, a response frequency of a torque sensor is set to 20 Hz or more and a frequency band of torque control is set to 20 Hz or more. A steering system normally has a mechanical resonance point at frequencies of 15 to 20 Hz. In actual fact, the system is prone to vibrations if the frequency band of torque control is set to 20 Hz or more.
The other documents related to the invention disclosed herein are as follows:    Prior Art 7: Japanese Unexamined Patent Publication No.2001-275325;    Prior Art 8: Japanese Unexamined Patent Publication No.2003-61272;    Prior Art 9: Japanese Unexamined Patent Publication No. H6(1994)-227410;    Prior Art 10; Japanese Unexamined Patent Publication No. 2001-133343;    Prior Art 11; Japanese Examined Patent Publication No. S62(1987)-38579;    Prior Art 12; Japanese Unexamined Patent Publication No. H11(1999)-124045;    Prior Art 13; Japanese Unexamined Patent Publication No. 2000-289638;    Prior Art 14; Japanese Unexamined Patent Publication No. H8(1996)-91236;    Prior Art 15; Japanese Unexamined Patent Publication No. H10(1998)-278818;    Prior Art 16; Japanese Unexamined Patent Publication No. 2001-233234;    Prior Art 17; Japanese Unexamined Patent Publication No. H8(1996)-119132;    Prior Art 18; Japanese Unexamined Patent Publication No. H8(1996)-308198;    Prior Art 19; Japanese Unexamined Patent Publication No. 2001-151121;    Prior Art 20; Japanese Unexamined Patent Publication No. 2002-372469;    Prior Art 21; Patent Publication No. 2782254